Paper containing electroconductive pigment and use thereof

ABSTRACT

THE CONDUCTIVITY OF PAPER IS INCREASED BY INCORPORATING A CONDUCTIVE ZEOLITIC ALUMINOSILICATE WITH THE PAPER AS A COATING OR AS A FILLER. THE RESULTING PAPER IS USEFUL IN VARIOUS NONIMPACT PRINTING PROCESSES. FOR EXAMPLE. WHEN PROVIDED WITH A SURFACE COATING OF PHOTOSENSITIVE MATERIAL SUCH AS ZINC OXIDE, THE PAPER IS EMPLOYED IN DIRECT ELECTROPHOTOGRAPHIC COPYING.

UnitedStates Patent O 3,694,202 PAPER CONTAINING ELECTROCONDUCTIVEPIGMENT AND USE THEREOF Edgar W. Sawyer, Jr., 22 Nottingham Road,Edison, NJ. 08817, and Frank J. Dzierzanowski, 2 Norfolk Road, Somerset,NJ. 08873 No Drawing. Filed June 5, 1970, Ser. No. 43,951 Int. Cl. G03g7/00; D21h 3/66 US. Cl. 961.8 8 Claims ABSTRACT OF THE DISCLOSURE Theconductivity of paper is increased by incorporating a conductivezeolitic aluminosilicate with the paper as a coating or as a filler. Theresulting paper is useful in various nonimpact printing processes. Forexample, when provided with a surface coating of photosensitive materialsuch as zinc oxide, the paper is employed in direct electrophotographiccopying.

BACKGROUND OF THE INVENTION In the manufacture of directelectrophotocopying paper (such as Electrofax), a sized paper base stockis coated on both sides with a conductive layer or coating. One side ofthe paper is then provided with a photosensitive layer, usually aphotosensitive grade of zinc oxide. Normally the photosensitive layeralso contains dyes to extend the spectral sensitivity of thephotosensitive layer to a desired range.

The light-sensitive coating is charged by exposed the sheet to a coronadischarge. The charged sheet is imaged by exposing the sheet toradiation from various sources such as a vidio tube, projectedoriginals, etc. The portions of the charged photosensitive surface whichhave been exposed to light discharge through the conductive resin layer;the portions which are not exposed to the light retain their charge. Theexposed sheets are then treated with a toner, which comprises finelydivided pigmented resin particles. Usually the toner isapplied in theform of a colloidal dispersion in mineral spirits. When treated with thetoner, the charged areas on the sheet selectively pick up the pigmentedresin, thereby developing the latent image. To set the pigmented resinon the sheet, the sheet is heated to drive off the solvent.

In present electrophotocopying paper the conductivity of the papersubstrate is increased by coating the paper with a conductive resin,specifically a polycationic resin having a high charge density. Indirect electrophotocopying paper, the conductive resin coating under thephotosensitive coating functions to dissipate the electrical charge fromthe irradiated areas of the photosensitive coating. It also effectsdecay (dark decay) of the charges on the photosensitive layer.

The conductive resin coating on the reverse side of the sheet effectscharge decay and it also aids in dissipating charge during irradiation.In addition, this conductive coating functions to prevent smudging inlarger white discharged areas on the photosensitive coating during tonerpickup. When a wet toner system (e.g., a toner containing mineralspirits) is employed, the resin coating on the reverse side of the sheetmay also function as a barrier to prevent penetration of the tonersolvent into the sheet.

Electrically conductive paper is also required in other nonimpactprinting processes, including certain electrographic processes which donot use light energy to develop an image and processes in which staticcharges are not involved.

Among the other agents suggested to increase the conductivity of paperfor use in various nonimpact copying processes are humectants,hygroscopic salts, polyanionic ice resins, graphite and metal powderssuch as powdered aluminum. See an article by Vaurio and Fird,Electrically Conductive Paper for Nonimpact Printing, TAPPI 47, No. 12,163A166A, December 1964. The humectants, hygroscopic salts andpolyanionic resins generally function satisfactorily at relativehumidities of 20 percent. However, at lower relative humidities they areunsatisfactory. Graphite and metallic powders function satisfactorilyover an range of relative humidities since the conductivity of suchmaterial is independent of humidity, but they are objectionable fordirect electrophotographic paper because of their color and theirweight. Aluminum foil provides an ideal substrate with regard toconductivity regardless of relative humidities and it is obviously abarrier against the penetration of solvent. However, the shortcomings ofaluminum foil as a base for the manufacture of directelectrophotocopying paper are obvious.

Thus, to date the commercially successful materials for impartingconductivity to the base sheet of direct electrophotocopying paper arecertain conductive cationic resins.

The cationic conductive resins, however, have the followingdeficiencies:

(1) They are expensive;

(2) They must be dried on the base sheet under carefully controlledconditions. If dried insufiiciently, the coatings become tacky at highrelative humidities. If overdried, the resins a relativelynon-conductive at low relative humidity;

(3) The resin coating on the reverse side of the sheet is generallytacky;

(4) The resins have an offensive odor;

(5) The resins yellow on aging;

(6) The conductivity of the resins leaves something to be desired.(Typically the surface resistivity of the resincoated paper is from 2x10to 2x10 ohm.)

THE INVENTION A general object of the invention is to provide novelconductive paper.

Another object is to increase the conductivity of paper by including inthe paper, as a coating or as a filler, certain zeolitic aluminosilicatepigments.

A specific object is to provide improved composite paper for directelectrophotographic copying.

Stated briefly, the presented invention resides in the use of finelydivided naturally-occurring or synthetic zeolitic aluminosilicatepigments which exhibit relatively high conductance without externalexcitation as fillers or coatings to increase the conductivity ofcellulosic paper. Suflicient zeolitic pigment is used to provide a sheethaving a surface resistivity with the range of 1x10 to 1 10 ohm atrelative humidities within the range of 7 percent to percent. (Aprocedure for testing surface resistivity is described in ASTM D256-46).

The term zeolite as used herein refers to an aluminosilicate which isbuilt up of linked $0., and A10 tetrahedra which form a negativelycharged network which contains channels running therethrough. In mostzeolites, water molecules occupy the channels when the zeolites are inhydrated form. The charge of the network is electrically balanced bycations which lie between the channels and are mobile in the sense thatthey can move from the spaces they occupy and be replaced by othercations of suitable size. In the case of a sodium aluminosilicatezeolite there is an exchangeable sodium ion for every aluminum atom intetrahedral coordination.

One preferred class of zeolitic zeolites includes natural and synthetichydrated crystalline zeolitic aluminosilicates containing mobile cationssuch as, for example, H+, NHJ, Ca++, Mg++, Na K' Li+.

Another preferred class of silicates includes synthetic amorphousprecipitated hydrated zeolitic sodium alumino silicates of the typedescribed in U.S. 2,739,073 and U.S. 2,848,346 to Bertorelli. The sodiumin these zeolites may be substituted with other cations such as thoselisted here inabove.

In one important embodiment of the invention, a paper base sheet whichis coated or filled with conductive zeolitic pigment to increase itsconductivity is provided with a surface layer of photsensitive materialsuch as zinc oxide to provide improved direct electrophotographiccopying paper.

PRIOR ART British Pat. 1,092,600 describes unsuccessful attempts toimpart antistatic properties to various synthetic resins by means ofsmall amounts of natural and synthetic zeolites. The zeolites alone hadno antistatic effect and cationic surface active agents had to beemployed with the zeolites.

U.S. 3,063,784 to Etchison suggests reacting nylon with a specific typeof clay (montmorillonite) to prevent static charge buildup and drysoiling. Montrnorillonite clay is not a zeolite within the scope of theinstant invention.

DISCUSSION The electrically conductive paper base of the presentinvention is especially useful as an element of copying paper for thedirect electrophotographic process (Electrofax). In addition to reducingthe surface resistivity of the paper substrate, the zeolite pigments mayalso impart a desirable white opaque appearance to the copying sheets.Such as appearance is not obtainable when cationic polymers, metallicpowders or graphite are employed to increase the conductivity of thepaper. Furthermore, the zeolites have the advantage over cationicpolymer of be ing conductive under a wider range of conditions oftemperature and humidity.

Reference is made to U.S. 3,052,539 to Greig and U.S. 3,121,008 to Joneset al. for details as to the preparation of photoconductive insulatinglayers of zinc oxide and insulating resin binder for use in thepreparation of direct electrophotographie paper. The photoconductivezinc oxide layer is normally sensitized with a dye such as one of thedyes enumerated in U.S. 3,121,008 to Jones et al. or U.S. 3,052,040 toGreig.

Paper base which has been rendered electrically conductive by means ofthe presence of zeolitic pigments, in accordance with the presentinvention, may also be employed in any of the nonimpact printing methodsdescribed in the article by Vaurio and Fird (supra). For example, thezeolites may be used in Timemark and Teledeltos type papers. In suchpapers an image is formed by causing an electrical discharge from a wirestylus to pierce an insulating film and expose the electricallyconductive paper. This printing method may be called electrography. Whenused in such paper the insulating film may have an intense color toprovide a contrast with the white image when the zeolite is exposed. Thezeolites may be used in similar paper in which the electricallyconductive base sheet carries a white nonconductive coating capable ofstoring a latent image formed as an electrostatic charge (e.g.,Videograph electrography paper) As another example, the conductive papermay be employed as the image receiving substrate for printing byxerography, such as the process described in U.S. 3,121,- 006 toMiddleton et al. Sheets coated on one side (nonima-ge receiving side)with zeolitic pigments are useful for indirect Xerox copying since thesheets will be free from static charges prior to, during and after imagereception.

The crystalline aluminosilicate zeolites used in carrying out thisinvention are electrically conductive without external excitation over awide range of relative humiditiese.g., from percent to 85 percent RH.

4 Generally, useful aluminosilicate zeolites are defined by the formula:

Mz o Z wherein M is cation such as hydrogen, ammonium, alkali metal,alkaline earth metal and combinations thereof; N is hydrogen or sodium;n is the valence of 'M; x and y are positive numbers which vary with thespecies of zeolite and fall within a definite range for each species. asusually varies from 2 to 15 and y from 1 to 10.

A preferred class of zeolites has a rigid, three-dimensional crystallineframework of SiO and A10 tetrahedra with pores of uniform diameterwithin the range of 3 to 15 angstrom units. These zeolites are known inthe art as zeolitic molecular sieves and have SiO /Al -O ratios (xvalues) from 2 to 6. Species are designated by letters, e.g., Type A(U.S. 2,882,243), Type X (U.S. 2,882,244), Type Y (U.S. 3,130,007). Tobe useful in the practice of the invention the sieves must be inhydrated form, e.g., N is hydrogen and the 1 values range from 2 to 9,and the sieves must contain mobile, ion-exchangeable cations. Aso-called decationized sieze (see U.S. 3,130,006) is not suitable.

Also within the scope of the invention are natural crystalline zeolitesand their synthetic counterparts. Examples an analcite, erionite,cha'bazite, faujasite (similar to synthetic type X and Y), natrolite,thomsonite, laumonite, scolecite, edingtonite, phillipsite, gmelinite,levynite, mordenite, etc. Synthetic counterparts of those naturalzeolite may be employed. Excellent results have been obtained withsynthetic sodalite of the approximate formula Other conductivecrystalline hydrated zeolites containing exchangeable cations are theclay minerals attapulgite and sepiolite. The pure minerals may be usedor the clay may be employed after removing grit and gross impurities.

The precipitated zeolitic pigments of Bertorelli Pat. 2,739,073 consistessentially of oxides of alkali metal, aluminum and silicon in a molarratio of SiO /Na O of at least 4/1 and have a molar ratio of Na O to A10 of about 1/ 1. The zeolites are obtained in extremely small particlesizes (finer than /2 micron) by reacting at low concentrations diluteaqueous solutions of alkali silicate and an aluminum salt of a mineralacid. The pigments obtained contain about 10 percent water (after beingdried at C.). To be useful in practice of this invention, at least asubstantial proportion of this combined water must be present with thezeolite. Thus the precipitated amorphous zeolites should not beoverdried.

The zeolites of Bertorelli patent U.S. 2,848,346 are obtained by amodification of the precipitation process of U.S. 2,739,073 wherein thezeolite is precipitated in the presence of pre-precipitated silica. Thepigments have a Na O/Al O mole ratio of about 1 and a Si-O /Na O moleratio within the range of about 4 to 14. These zeolites contain about 10percent water when dried at 110 C. and must contain water of compositionwhen employed in the practice of this invention.

The zeolitic molecular sieves and synthetic zeolites are normallyproduced in sodium form. Naturally occurring zeolites usually containsodium, calcium or barium or combinations thereof or with magnesium asthe principle exchangeable cations. These natural zeolites or thesynthetic zeolites may be ion-exchanged before use.

The zeolites should be free from grit (particles plus 325 mesh Tyler).The particles are preferably finer than 10 microns (equivalent sphericaldiameter). Particles within the range of about 0.3 to 5 microns areespecially desirable.

In an embodiment of the invention, the zeolite is incorporated with thepaper as a filler. This may be accomplished by the conventional methodof adding the finely divided zeolite particles to cellulose pulp andforming the mixture into a sheet by means such as a Fourdrinier screen.Filler retention aids such as alum may be employed. Alternatively, thezeolite can be incorporated by spraying a slurry containing the zeoliteonto the paper Web at or near the wet end of a papermaking machine.

When employed as filler the zeolite is generally present in amountwithin the range of about percent to percent, based on the dry weight ofthe paper. When used in amount less than about 10 percent, the effect ofthe zeolite may be minimal or insufficient. When used in amount inexcess of 15 percent, certain properties of the paper such as strengthmay be adversely affected. The filled paper is normally sized, dried,calendered, etc.

To produce zeolite-filled direct electrophotographic copying paper, aphotosensitive coating (e.g., a coating described in Greig patent U.S.3,052,539) is applied to one side of the zeolite-filled paper base.Preferably a barrier coating, e.g. a coating of starch or polyvinylalcohol, is applied to the reverse side.

It is also within the scope of the invention to include the zeolite inpaper as a coating. This coating may be applied to uncoated paper or thecoating may be superimposed on a coating of clay or the like. When usedas a coating, it may be desirable to dilute or blend the zeolite withkaolin clay or calcium carbonate because coating compositions containingsuch mixtures exhibit better coating characteristics than docompositions containing a zeolite as the sole pigment.

The conductive pigment may be incorporated with the paper as a coatingon one side or both sides of the paper sheet. The coating is normallyapplied to the sheet as an aqueous composition containing conventionalcoating adhesives (such as starches, e.g., oxidized, chemicallymodified, etc., protein, latex, starch-latex, protein-latex, etc.).

It is within the scope of the invention to provide sheets containing acoated layer of electrically conductive zeolitic pigment underlying aphotosensitive coating and a barrier coating, e.g., starch or polyvinylalcohol, coated on the underside (reverse side) of the sheet. It is alsowithin the scope of the invention to coat both sides of paper sheet withconductive zeolitic pigment, apply a photosensitive layer of zinc oxideover one of the coatings and, if necessary, apply a barrier coating overthe conductive coating on the underside.

The use of conductive white or oil-white zeolitic aluminosilicatepigments, in accordance with the present invention, represents animprovement over the use of polycationic resins in many respects. Thecoated sheets may have better color and the sheets will not yellow withage. The conductivity of the paper sheets may be increased at a fractionof the cost entailed when using the resins. An important advantage ofusing zeolites to decrease the electrical resistivity of paper is thatsheets containing conductive zeolitic pigments have appreciably betterconductivity at low relative humidity than those treated with cationicresins. Problems encountered when cationic resins are overdried orunderdried are obviated. Furthermore, sheets formulated with thezeolitic silicate pigments are free from objectionable odor and arenontoxic.

The paper stocks employed to produce the base sheets for nonimpactprinting will vary, of course, with the printing process. [Paper stockssuitable for direct electrophotographic paper containing photosensitivezinc oxide are described in an article by Joseph Savit, -A {Review ofPhysical, Chemical and Electrical Characteristics of Liquid TonerConductive Base Papers, TAPPI, vol. 52, No. 10, October 1969.

Surface resistivity values for various types of paper used in nonimpactprinting appear in the article by Vaurio and Fird (supra). According todata in this article, at 12 percent R. H., the surface resistivityValues for sulfite bond (Aquapel size and rosin size), kraft(impregnated, and ground wood) exceeded 3.75 10 ohm. By filling suchpapers with hydrated zeolites containing mobile cations (or by coatingthe paper with these zeolites or mixtures thereof with diluents such askaoline clay), the surface resistivity of such paper can be decreased tovalues at least comparable to those obtained with commercial cationicresins.

Example I This example demonstrates the desirable electrical propertiesof zeolites within the scope of the invention.

Volume resistivities of cakes of sodium zeolite Y and Zeolex 20 weremeasured by the procedure substantially as described in US. 3,011,918 toSilvernail et al. Zeolex 20 is an amorphous precipitated hydrated sodiumaluminosilicate of the type described in the Bertorelli patents. Volumeresistivity of starch coatings containing these zeolites were alsomeasured. The coatings were made by applying an aqueous coatingformulation containing parts by weight zeolite to 25 parts oxidizedstarch to aluminum foil and drying at 250 F. for 15 minutes. Similarcoatings for aluminum foil were prepared containing 25 parts by weightzeolite, 25 parts by weight of No. 1 grade coating kaolin clay as adiluent and 25 parts by weight starch. For purposes of comparison, DowCationic Resin ECR 34 was applied on another piece of foil. The resin isa polymer of vinylbenzyl quarternary ammonium compound of the typedescribed in U.S. 3,011,918 (supra).

In all cases measurements were made at 156 F. on samples oven dried at250 F. for 15 minutes and at 250 F. on samples oven dried at 250 F. for4 hours.

The volume resistivities of cakes and coatings containing zeolite ormixtures of zeolites with a clay diluent were less than 4x10 ohm-cm. formeasurements made at 158 F. The resin coating had a higher volumeresistivity (158 F.) of 3.6 10 ohm-cm. At 250 F., sodium zeolite Y andcoatings containing zeolite Y or Zeolex 20 without a diluent had volumeresistivities below 4x10 ohm-cm. The coatings with clay diluent hadvolume resistivities of the order of 10 ohm-cm. The cationic resindecomposed after being heated at 250 F. for 4 hours and the resistivityat 250 F. could not be measured.

The data in this example therefore show that a synthetic crystallinezeolite (sodium zeolite Y) and a synthetic amorphous zeolite (Zeolex)were at least as effective as a commercial cationic resin in impartingelectrical conductivity to paper when coated on the paper.

Example II This example illustrates the preparation of electricallyconductive zeolite-filled paper.

A conventional method for filling the paper was used. Chemical fiber waspulped in water at about 1 percent solids. To portions of the pulp,Zeolex 20 and sodium zeolite Y were added, followed by addition of alumas a retention aid. The slurries were drained over a forming screen inconventional manner to form sheets. The sheets were dried andcalendered. The filler contents of sheets were computed afterdetermining the ash contents.

It was found that sheets containing 9 percent to 15 percent of thezeolites were more conductive than sheets of the same paper coated withthe cationic resin used in Example 1.

Example III The procedure of Example I was repeated using Attagel 50(fluid energy milled, degritted, crystalline hydrated attapulgite clay)as the zeolitic material. The coating contained 25 parts by weightAttagel, 75 parts by weight of No. 1 grade kaolin clay and 25 parts byweight of the starch. The volume resistivity of the coating was similarto that of coatings containing sodium zeolite Y or Zeolex 20.

Example IV Aluminum foil was coated at 50 percent solids with aball-milled mixture containing 25 parts by weight synthetic crystallineso'dalite (3Na O.Al O .2SiO .2NaOI-I), 75 parts by weight No. 1 kaolincoating clay and 25 parts by weight cornstarch. The volume resistivitywas 1.6 X 10 ohm-cm. at 5 percent R.H.

Another piece of foil was coated at 50 percent solids with a coatingcomposition containing 100 parts by weight of synthetic sodalite(neutralized with hydrochloric acid) and 30 parts by weight cornstarch.The volume resistivity was 3.1 X 10 ohm-cm. at 5 percent R.H.

1W6 claim:

1. A photoconductive element for direct electrophotographic printingcomprising an electrically conductive base sheet which consistsessentially of cellulosic paper filled with finely divided particles ofan electrically conductive zeolitic aluminosilicate containing mobileoations, said particles being present in amount such that said filledbase sheet has a surface resistivity within the range of 1x10 to 1x10ohm at relative humidities Within the range of 7 percent to 85 percent,and on a surface of said electrically conductive base sheet a continuouslayer of photoconductive zinc oxide and an insulating film-formingorganic resin binder therefor.

2. A photoconductive element for direct electrophotographic printingconsisting essentially of a cellulosic paper base sheet materialcarrying on at least one surface thereof a continuous coating of azeolitic aluminosilicate containing mobile cations in quantitysufficient to provide a coated base sheet having a surface resistivitywithin the range of 1 10 to 1X10 ohm at relative humidities within therange of 7 percent to 85 percent, and on one outer surface of said papera continuous layer of photoconductive zinc oxide and an insulatingfilm-forming organic resin binder therefor.

3. The photoconductive element of claim 1 wherein said zeoliticaluminosilicate is a crystalline molecular sieve having uniform poreopenings within the range of 3 to 15 Angstrom units.

4. The photoconductive element of claim 1 wherein 8 said zeoliticaluminosilicate is a synthetic amorphous precipitated hydrated sodiumaluminosilicate.

5. The photoconductive element of claim 1 wherein said zeoliticaluminosilicate is synthetic sodalite.

6. The photoconductive element of claim 2 wherein said zeoliticaluminosilicate is a crystalline molecular sieve having uniform poreopenings within the range of 3 to 15 Angstrom units.

7. The photoconductive element of claim 2 wherein said zeoliticaluminosilicate is a synthetic amorphous precipitated hydrated sodiumaluminosilicate.

8. The photoconductive element of claim 2 wherein Said zeoliticaluminosilicate is synthetic sodalite.

References Cited UNITED STATES PATENTS 3,295,967 1/ 1967 Schoenfeld96-1.5 2,739,073 3/ 1956 B'ertorelli 106-306 X 3,266,973 8/1966 Crowley1621 81 C 3,294,535 12/ 1966 Powers 96-49 2,918,399 12/1959 Eichmeier162-181 C 3,063,784 11/1962 Etchison 8-1155 2,637,651 5/1953 Copley96-1.4 X

FOREIGN PATENTS 1,092,600 11/ 1967 Great Britain.

449,713 5/1969 Japan 96-1.5

GEORGE F. LESMES, Primary Examiner R. E. MARTIN, JR., Assistant ExaminerUS. Cl. X.R.

, Patent No. 3,694,202 Dated September 26, 1972 Inventor) Edgar W.Sawyer, Jr. and Frank J. Dzierzanowski It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 1 line 7, insert after the inventors names and addressesassignors to Engelhard Minerals & Chemicals Corporation, Township ofWoodbridge, New Jersey line 32, "exposed" should read exposing Column 2line 9, "over an" should read. over a Column 4 line 26, "an" should readare Column 5 line 75, "kaoline" should read kaolin Signed and sealedthis 24th day of April 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTT'SCHALK Attesting Officer Commissionerof Patents FORM PO-1050 (10-69) USCOMM-DC 6O376-P69 u.s. GOVERNMENTPRINTING OFFICE: 1989 o-aee-su

